Coherent Optical Communication

Abstract

Virtually all fibre transmission systems up to the present have used simple intensity modulation of the transmit laser, most frequently in the form of binary ON/OFF modulation. At the receiver, incoming optical power has been converted directly to photocurrent in a photodetector. Such receivers typically operate with sensitivities of 2000 photons/bit for 1E-9 bit error rates at 1300 or 1500nm wavelength, approximately 20dB away from the theoretically ideal photodetector performance. The only technique that has succeeded in closing this gap to date is that of the coherent system, using an optical local-oscillator at the receiver to operate with a photo-detector mixer in heterodyne or homodyne mode. Apart from the receiver, the transmitter design must be radically changed also and, ultimately, the technology seems likely to lead to new ways of using light for communications.